My 85' Vette has FORD 24# injectors rated at 39.15 PSI.

My fuel rail has 38 PSI prime pressure and 28 PSI Idle pressure.

My vacuum at the nipple on the FPR is measured at 15 in/Hg.

This is not an adjustable FPR, but does look newer than what was stock.

By the research this car should have 48 PSI prime pressure.

Why do I only have 38 PSI primed?

Why is only 10 PSI lost at idle when there is 15 in/Hg on the line?

Why does my fuel pressure increase to prime pressure when I shut off the car completely??

By those numbers my car is flowing 23.16# at WOT and 20.29# at idle, which is close to the stock fuel flow.

Fuel pump going bad or someone put in a wrong pump?
FPR going bad, or incorrect spring size?

I was gonna get a AFPR to set primed to 43PSI with a 15 in/Hg would make idle 20.29# flow and WOT 25.15# flow. Seeing this I should have no problem leaving it the way it is unless something is gonna to fail.

Any help to these questions would be great!

 

I am not sure how you came up with those numbers, maybe you know something that I don't. Are you subtracting vac numbers in inches from fuel pressure in pounds?
Your Ford injector flow rate is rated at a different fuel pressure than GM ones, making them somewhat larger in a GM fuel rail. Most L98's have 22 pound injectors set at 40 pounds or so. The stock 24's in a 85 Vette are set at 35- 36 pounds (I think). You need to check pressure at idle, and while driving. I am thinking you need to be at about 33-35 pounds with the Fords. Most GM's pop up in pressure when the engine is shut down, this is normal. Are you having driveability issues? I am sure some of the injector experts on here will chime in.

 

Yes I am just subtracting the vacuum from the pressure. I know the FORD and GM injectors were rated at a different pressure. The injectors I have in there are FORD 24# at 39.15 PSI.

She sometimes stumbles a bit after WOT let off and runs a bit rich at WOT as well. Only tell tale signs are the dark exhaust tips and stumbling after WOT let off.

She had a smog pump problem where it was stuck open and tricking the car into thinking its lean causing a super rich condition. I since have removed all the smog system and put a pulley in its place and routed the FPR directly to the intake manifold.

The original vacuum line saw only 8 in/Hg while this line directly off the intake manifold sees 15 in/Hg.

Possibly that my plugs are fouled out from the rich running?

 

I went out to verify my problem further.

I discovered that the rough idle becomes more prominent the longer I idle.

First 3 min I had a rock solid 550 RPM Idle. Then it got rougher and rougher over time. It almost sounded like one of my cylinders stopped igniting.

The RPM dropped slightly to 500 RPM and began to hunt for idle. Giving it gas will makes the roughness go away after a few seconds and it fells like something is releasing as I'm getting a small extra bit of power when it happens.

11.7 V is lowest it got on voltmeter in dash. Highest was 17.4 V on a WOT run.

It one time got to 10.2V on the dash volt meter but I had the stereo cranking tunes.

 

I'll try to answer your questions. I tried to put just my answers in italics but it looks like possibly everything went italics, hope you can read them OK.

 

First, the math is all wrong and off course.

next, you need an adjustable FPR and a good test gauge.

Heres what you NEED for relaible performance...

40+ psi on the gauge at key-on.
At start up just slightly (2-5 psi) less pressure at idle.

Only 2-5 psi drop as engine is revved with INSTANT recovery to the original pressure reading.

A steady needle. No flicker, no bouncing.

AT key OFF, eng shut down,. little or NO drop in fuel pressure for 20 minutes. Less than 5 psi drop after 1 hr.
Less than 10 psi after 2-3 hrs.
The system should still have 20 psi or more after 6-8 hrs. There should still be pressure the next morning.

The pressure on the gauge is a reflection of whats being used by the engine....as the eng revs the press drops as fuel is used UNTIL the pump can replace that amount which takes about 1-second.
The system needs to hold pressure when the pump is OFF. This proves that the inj are not loose and leaking. The system is designed to leak back into the tank thru the pump BUT the pump has a check valve so this does not happen quickly.

The regulator is vac actuated with a spring to apply resistance. If that spring gets weak the natural eng vac can pull the diaphram too far and cause lowered pressure to the rails. The regulator can leak internally and loose fuel to the return side of the system. The diaphram can leak and let the eng suck wet fuel into the vac line and cause misfire, rich run/idle and poor performance. A FSM has all the test proceedures to track the different possibilities. The best investment that a C4 owner can ever make...

AT this point you need to "prove" the problem and change the filter, retest pressure, then change the regulator and retest pressure. Then you have proven the pump is not supplying the pressure and needs to be replaced. Pumps get weak and fail when they have been run with a low tank level and/or too much trash in the system. The screen/sock on the pump gets clogged and restricts what the pump can suck in and then limit what the pump can push out.

20 psi is not acceptable at ANY eng speed. You're looking for 40+ under all conditions. 20 is barely enough to run. Forget the vac at the reg nipple...thats going to change and it means nothing unless you can do the math to see what the pressure is opposed to the spring pre-tension. As long as vac is present, the regulator will function. The spring is far more important than the vac.
Get a regulator and new filter and go from there...IF the problem persist.

 

Did'nt see this before....

heres 1/2 the problem. You can't do that to a computer controlled engine without doing a bunch of other things.
Certain vac functions are "ported vac" and are not supposed to have the full vac available at all times. That does damage to delicate things like diaphrams and seals. You desparately need a FSM.

 

Thank you for all the responses and help. I'm going out of town for a month but when I get back I'll grab a FSM and a new regulator and test the fuel system out.

 

An FSM is a GREAT idea.

But, before you run off an buy parts....

Test the fuel system and make sure you have the right pressure. Don't add or subtract anything. Get a fuel gauge, hook it up, tape it to the windshield and drive around. We need to know what it is doing at idle, neutral rev, steady driving, and WOT. We need pressure and any sort of oscillations on the needle. We need EVERYTHING.

Those Ford injectors are probably flowing too much.

Watch this video by our good friends at FIC

You'll learn something, I can guarantee it. He also goes over the Ford injectors.

 

Please take into account I am doing this with the battery fully disconnected from the car after pump is primed. I'm using a 50 dollar Autozone hand held fuel pressure tester 0 - 100 PSI. When I first attached it to the car this morning there was 0 PSI on the rail. This was approximately 23 hours of sitting time.

I primed the fuel rail with the pressure gauge connected - 38 PSI
Within 3 min after turning the car off it dropped to 35 PSI.




Ill go check in in a few again and update this thread. All I can really do right now since I leave tomorrow morning.

 

Waiting... but 3 PSI in 3 mins?
that low prime pressure?

Bad pump or leakdown. Has anyone tried to test FP at the fuel filter? I would try that to half-split the system.

If you get high PSI at the filter, it's leakdown or bad regulator. If you get 38 with the pressure drop like you are now, then it's the pump.

Either way, I'll wait for your post in an hour or so. I want to hear what the PSI is then.

My money is on.... 20 PSI!
Any takers?

 

2:38PM to 4:57PM 35PSI down to 29.5PSI

About 2 PSI leak down an hour so far.

All symptoms are point to bad pump or clogged line. Most likely the fuel pump seeing the bad idle starts when it get to 200+ coolant temp. I'll pull fuel filter off and check it before I buy any parts.

I also need to put in a pulse dampen cause she beat boxes when its cold outside for few 5 min.

Updates in about an hour or so.

 

4:57PM was at 29.5PSI
7:20PM sitting at 25 PSI

4.5 PSI lost in about an hour and a half, about 3 PSI and hour now.

 

7:20PM was at 25 PSI
11:10PM is at 19 PSI

That's a 1.71 PSI loss an hour.

So this proves my injectors are fine? Do I just start troubleshooting the lack of primed fuel pressure now?

Could the lack of fuel pressure be due to a faulty alternator? My alternator is not strong enough to power the car at Idle unless I have my stereo completely off and still she drops really low. I really hinky voltage reading all the time low as 10.7V and as high as 17.4V I have seen on the dash.

You nailed it! "My money is on.... 20 PSI! Any takers?"

 

So I have a good amount of time to do research while I'm TDY. I was just running some numbers and please correct me if I'm wrong.

If I'm getting 38 PSI prime pressure while on those injectors makes about 23.65 Lbs/Hr based solely on rail pressure.

Then why don't I just remove the vacuum all together and call it a day.

The ECU will adjust the BLM's to correct Idle and less than 50% throttle richness.

If I'm feeling froggy I could just get an adjustable FPR and set it to 39.15 PSI to get a smidgen more fuel if I need it. I assume it will only run slightly leaner in WOT and may improve performance. At Idle I could only hope that the ECU can adjust the BLM's enough to thwart any problems.

Your help is appreciated!

 

One test I haven't seen mentioned here is to disconnect the vacuum line at the FPR and read the fuel pressure. When you do that you see the pressure that the bypass in the fuel pump is working at. This is NOT a precision value, but should be over 50 PSI. If you get over 50 PSI then your pump and fuel filter are OK.

The ECM has built-in limits on how much compensation it can do for your modifications. If it crashes up against a BLM limit your engine will not run its best. The BLM system is pretty limited in my opinion. You're only going to be operating in a few cells. I doubt that it is even possible to get into some of those cells (3 & 12 especially and maybe 2 & 7 and 8 & 13). My car spends almost all its time in cells 10 & 15.

Setting the EPROM values to the correct injector flow rate will help a lot. As it is now, the ECM doesn't know you have increased the flow rate. You will run rich in open loop mode (WOT & low temperature). The ECM will do its best to compensate in closed loop mode but can hit the BLM limits. You really need to use a scan tool to see exactly what is happening.

One other problem with larger injectors that I have seen mentioned elsewhere is that the ECM has a minimum injector pulse width limit of 1.693 ms. If the ECM decreases the pulse width until it hits this limit you will have a rich idle, which can foul plugs and cause other problems.

There's more... The ECM does not know you have removed the smog pump. It compensates for the extra air in the exhaust manifolds by offsetting the O2 sensor reading. Since the smog pump is not there, it puts in the offset even though there is no extra oxygen present. That's one more error in your air/fuel ratio.

 

I thought the AIR system pumps all the time. The diverter valve would send it to the manifolds when the engine is cold. Once it hits closed loop and it can see feedback from the O2 sensor, air is sent to the cats? If so, where does this affect anything? The sensor is upstream from the cats and when it does get fresh air, it isn't?

 

I have my adjustable FPR set for 45 psi with the hose off. It seems like your FPR holds pressure. Not sure if it is the injectors or not. They are probably original or old. I would take them out for testing and cleaning every few years.

I tend to wonder if your fuel pump is clogged or crapped out. I have it prime to 45 and running about that, give or take a few psi at WOT.

 

The smog pump has 3 "modes", controlled by two solenoids (which are controlled by the ECM). Mode 1 (these numbers are arbitrary and I just made them up) is controlled by the "divert" solenoid and the other two are controlled by the "switch" solenoid.

Mode 1 is divert mode. The air is diverted to the outside world through the muffler that hangs down in front of the engine. The divert solenoid also sends the air to the switch solenoid for modes 2 & 3.

Mode 2 is A.I.R. (Air Injection Reaction) mode, where the air is injected into the exhaust manifolds right where they attach to the heads. This is meant to burn any raw fuel that doesn't get burned in the combustion chambers. When air is injected here it affects the O2 sensor reading, as I stated before.

Mode 3 is catalytic converter mode, where the air is injected into the back half of the main catalytic converter. The car runs almost all the time in this mode (like 90% - 95% of the time).

 

In mode 3, there should not be air interfering with the O2 sensor readings.

In Mode 2, it could but I thought that is in open loop where the O2 sensor readings were disregarded because it hasn't reached the right temps yet?

Or maybe there is a malfunction and it is in mode 2 constantly and blowing fresh air over a "live" O2 sensor and screwing up the readings?

I am kinda hazy on this since my AIR system "fell" off so long ago.